The present invention relates to a line filter to be inserted for use in a power source line of various kinds of electric devices.
A practical line filter of this type has been proposed as shown in FIG. 20 which comprises a metal case 5 of rectangular solid shape having a pair of input terminals 1, 2 and a pair of output terminals 3, 4, and a filter unit 6 having an electric equivalent circuit as shown in FIG. 21, the filter unit 6 being accommodated within the metal case 5. In this conventional line filter, the filter unit 6 is composed of one bypass-type condenser (capacitor) C1 for use to suppress the noise of normal mode, connected between a pair of lines 8a and 8b provided on a printed circuit board 7, and a pair of bypass-type condensers C2, C3 for use to suppress the noise of common mode, one condenser C2 connected between the line 8a and an earth line 9 formed on the printed circuit board 7 and the other condenser C3 connected between the line 8b and the earth line 9, the earth line 9 being soldered to the metal case 5 by means of solder.
The upper limit frequency of noise to be eliminated by the above line filters is normally restricted to about a maximum of 30 MHz, by existing residual inductance on lead lines 10'a, 10'b of the bypass-type condenser C3 for common mode noise, and on lead lines 10a, 10b of the bypass-type condenser C2 for common mode noise. This is because the residual inductance of lead lines 10a, 10'a acts to reject noise to be inputted into both bypass-type condensers C2 and C3 for common mode noise, while the residual inductance of lead lines 10b, 10'b acts to suppress noise being bypassed to the earth from the bypass-type condensers C2 and C3 for common mode noise.
Generally, noise which may be inputted into electric devices of various kinds through a power source line, or which may be outputted from the electric devices, is classified into two kinds: conduction noise and radiation noise. The conduction noise largely contains frequency components lower than 10 MHz which can be easily removed by the employment of the conventional line filter, but the radiation noise, as well higher harmonic waves of clock signals used in electric devices, often contains high frequency components of from 30 MHz up to several hundred MHz which are difficult to remove by means of the conventional line filter in any way. Therefore, a through-type condenser (feedthrough capacitor) is employed in place of each of the bypass-type condensers C2 and C3 for common mode noise, which is mounted on the metal case 5 to provide an equivalent circuit as shown in FIG. 22. The line filter with the through-type condensers can remove the high frequency components from the noise better than the conventional filter having bypass-type condensers, by decreasing the residual inductance which is formed on the lead lines in the conventional filter. However, the through-type condensers are expensive and increase the cost of the line filter to a large extent. In addition, if the line filter having through-type condensers C2' and C3' for common mode noise is used within a frequency range where the bypass-condenser C1 for normal mode noise functions as an inductor combined with a capacitor, all of the above condensers C1, C2' and C3' are caused to form an LC resonance circuit which reduces the function of the through-type condensers for removing the noise in the line filter.